Concentration graded carbon doped oxide

ABSTRACT

A process for forming an interlayer dielectric layer is disclosed. The method comprises first forming a carbon-doped oxide (CDO) layer with a first concentration of carbon dopants therein. Next, the CDO layer is further formed with a second concentration of carbon dopants therein, wherein the first concentration is different than the second concentration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to low k dielectrics, and moreparticularly, to a carbon doped oxide having a carbon dopingconcentration that is graded.

2. Background Information

As integrated circuit technology advances, integrated circuit devicesbecome smaller and smaller. This allows for greater speed and increaseddevice packing densities. Sizes of individual features, for example thetransistor gate length, on modern integrated circuits is shrinking toless than 50 nanometers. The resultant increase in packing densities hasgreatly increased the number and density of metal interconnects on eachchip.

The metal interconnects (which consist of conducting lines and vias)have become smaller, more complex, and more closely spaced. The smallersizes of the interconnect pitch leads to RC (resistance-capacitance)coupling problems which include propagation delays and cross talk noisebetween interlevel and intralevel conductors. RC delays thus limitimprovement in device performance. Additionally, fringing electricalfield effects near the metal lines may adversely affect performance ofthe interconnects.

Capacitance can be reduced by employing low dielectric constant (low k)dielectric materials to insulate the metal interconnect lines. Sincecapacitance is directly proportional to the dielectric constant of theinsulating material, the RC delay can be reduced when a low k materialis used. Various semiconductor equipment manufacturers have developedlow k dielectrics. One of the most promising low k dielectrics is thecarbon-doped oxide (SiO_(x)C_(y)H_(z)).

While carbon doped oxide (CDO) film has been found useful to reducecapacitance by lowering film density and polarizability of bonds, CDOfilm has poor thermal and mechanical properties. For example, CDOexhibits poor hardness, is susceptible to cracking, and has low thermalconductivity.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing aspects and many of the intended advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-section of a semiconductor substrateillustrating one use of a prior art carbon doped oxide film as aninterlayer dielectric.

FIG. 2 is a schematic cross-section view of a semiconductor substrateillustrating a carbon doped oxide film formed in accordance with thepresent invention used as an interlayer dielectric.

FIGS. 3-5 are schematic cross-section views of a semiconductor substrateillustrating a carbon doped oxide film formed in accordance withalternative embodiments of the present invention.

FIGS. 6-9 are schematic diagrams of chemical reactions suitable forforming carbon doped oxide.

DETAILED DESCRIPTION OF THE INVENTION

The present invention describes a method for forming a carbon dopedoxide (CDO) film having a graded concentration of carbon doping. In oneembodiment, the CDO film is utilized for interlayer dielectricapplications.

In the following description, numerous specific details are provided toprovide a thorough understanding of embodiments of the invention. Oneskilled in the relevant art will recognize, however, that the inventioncan be practiced without one or more of the specific details, or withother methods, components, materials, etc. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the invention.

Reference throughout this specification to “one embodiment”, “preferredembodiment”, or “an embodiment” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention. Thus,the appearances of the phrases “in one embodiment”, “in a preferredembodiment”, or “in an embodiment” in various places throughout thisspecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments.

Turning to FIG. 1, in a typical application, a CDO layer 105 isdeposited atop of a substrate 101. The term substrate 101 as used hereinmay include a semiconductor wafer, active and passive devices formedwithin the wafer, and layers formed on the wafer surface. Thus, the termsubstrate is meant to include devices formed within a wafer and layersthat overlie the wafer. Further, the CDO layer 105 is formed over metalconducting structures (also referred to as a metal interconnect) 103formed on the substrate 101. As noted above, the CDO layer 105 isespecially useful for it's low k dielectric properties. Thus, it isuseful in an intermetal or interlayer dielectric application to minimizecapacitance between various segments of the metal interconnect 103.

Further, typically, the CDO layer 105 is patterned and etched inaccordance with a desired via pattern. The carbon-doped oxide layer 105is etched to form vias 107 in the carbon-doped oxide layer 105. In theprior art, the CDO layer 105 has a homogenous concentration of carbondopants. This can be seen in FIG. 1, which includes a dopantconcentration chart that shows the relative level of dopantconcentration relative to the depth of the CDO layer 105. As seen, thedopant concentration is the same throughout the entire depth of the CDOlayer 105.

In accordance with the present invention, the carbon dopantconcentration is graded. Specifically, in those regions of the CDO layer105 that are proximal to the metal interconnect 103, a high carbondopant concentration is provided. This lowers the k value of the CDOlayer. In those regions of the CDO layer 105 that are distal to themetal interconnect 103, e.g., between the vias 107, a relatively lowcarbon dopant concentration is provided. This increases the hardness andincreases cracking resistance and thermal conductivity of the CDO layer105.

Specifically, turning to FIG. 2, a CDO layer 205 has a gradedconcentration of carbon dopants. In this particular example, the dopantconcentration is highest near the metal interconnect 203. Where the CDOlayer 205 surrounds the vias 207, the carbon dopant concentration isless. Further, although the dopant concentration is shown to decreaselinearly from bottom to top of the CDO layer 205 (referred to as alinear concentration profile), a discontinuous stepped decrease ofcarbon dopants is also contemplated to be in accordance with the presentinvention.

The advantage of such a CDO layer 205 is that proximal to the metalinterconnect 203, a high carbon dopant concentration is present. Thislowers the dielectric constant, thereby reducing electrical field fringeeffects and capacitance. Proximal to the vias 207, where capacitanceissues and fringe effects are not as pronounced, the carbon dopantconcentration is lower. In the region proximal the vias 207, the CDOlayer 205 has enhanced hardness, greater resistance to cracking, andgreater thermal conductivity, relative to the CDO layer 205 proximal themetal interconnect 203.

The present invention can also be applied to dual damascene typestructures. For example, turning to FIG. 3, a multilevel metalinterconnect and via structure is shown. The first level of the metalinterconnect includes a first metal layer (M1) 301 that has beendeposited onto a substrate. Formed between the first metal layer M1 301is a first dielectric layer 303. The first dielectric layer 303 may be,for example, a carbon doped oxide, a fluorine doped oxide, or the like.In accordance with dual damascene processes, a carbon doped oxide layer305 is deposited above the first dielectric layer 303 and the firstmetal layer M1 301.

As seen in FIG. 3, for this dual damascene application, the carbondopant concentration of the carbon doped oxide layer 305 progressivelyincreases as the CDO layer 305 is deposited. Therefore, a lowerconcentration of carbon dopant is present during initial deposition ofthe CDO layer 305 and a higher carbon dopant concentration is providednear the completion of the CDO layer 305. After the CDO layer 305 isdeposited, patterning and etching steps are performed to form dualdamascene openings 307 formed in the CDO layer 305. Further, typically asecond metal layer is deposited into the openings 307. In this manner,the via and the second metal layer can be deposited at the same time.

By having the lower carbon dopant concentration adjacent to the viaportion of the opening 307, enhanced hardness, thermal conductivity, andresistance to cracking is provided. Note that in the upper region of theopenings 307 (carrying the second metal interconnect structure), ahigher carbon dopant concentration is provided. This reduces thedielectric constant and provides the advantages noted above. Thus, inthis embodiment, the higher dopant concentration is near the surface ofthe CDO layer 305 and the lower concentration of dopant is provided nearthe bottom of the CDO layer 305.

In an alternative embodiment, referring to FIG. 4, the CDO layer 305includes a high dopant concentration initially, followed by a lowerdopant concentration, finally followed by a high dopant concentrationnear the top of the CDO layer 305. This dopant concentration profile ishelpful in reducing electric field fringe effects arising from the firstmetal layer M1 301. This dopant profile is referred to as a concavenonlinear dopant concentration.

Still alternatively, referring to FIG. 5, the dopant concentration ofcarbon in the CDO layer 305 may be higher in the middle region of theCDO layer 305 compared to the top and bottom regions of the CDO layer305. Thus, at the top of the first metal layer 301, a high carbonconcentration is provided. This helps to reduce electric field fringeeffects near the first metal layer 301. Next, moving upwards from thefirst metal layer 301 towards the second metal layer, the carbonconcentration is lowered in the via region of the dual damascene layer.Then, the carbon concentration is increased as the second metal layer isapproached. Finally, a low carbon concentration is provided near the topof the second metal layer. Indeed, the top of the carbon doped oxide maybe almost completely depleted of carbon. This top layer can then be usedas a hardmask for the CDO layer. This dopant profile is referred to as aconvex nonlinear dopant concentration.

In one embodiment, the carbon dopant concentration ranges from 1-20% (byatomic mass) for the carbon dopant. Thus, for low carbon dopantconcentration regions, 1% carbon dopant is provided. For high carbondopant regions, up to and over 20% carbon dopant concentration may beused. However, it can be appreciated that other dopant concentrationsmay be used, customized for the specific application.

The formation of variably doped CDO is described in relation to FIGS.6-9. While there are many methods of depositing carbon doped oxides, theamount of carbon dopant present in the carbon doped oxide can be variedby modulating the flow rate of the process gases, or by modulating theRF power, pressure and temperature in accordance with known techniques.

For example, turning to FIG. 6, the chemical reaction may constitute theflowing of dimethyldimethoxysilane (DMDMOS) with a diluting agent suchas helium. By flowing these gasses into the process chamber with theapplication of energy to generate a plasma, a CDO layer can be formed.Alternatively, turning to FIG. 7, trimethylsilane (3MS) can be flowedinto the process chamber with one of the following gasses: N_(o), O₂,O₃, CO, or CO₂. This will also result in a CDO layer. Stillalternatively, turning to FIG. 8, tetramethylsilane (4MS) can be flowedwith any of the above oxygen containing gases to generate a carbon dopedoxide layer. Still alternatively, tetramethylcyclotetrasiloxine (TMCTS)may also be used to form the CDO layer. The resulting CDO layer fromthese processes has a chemical composition of SiOC_(x)H_(y).

It should be noted that while four specific examples of gases that canbe used to form the CDO layer have been described above, numerous othertechniques may be used to form CDO. The present invention teaches that,contrary to the prior art, a CDO layer can be formulated having avariable carbon dopant concentration. While in one embodiment, thecarbon dopant concentration can be varied in situ to improve throughput,as well as to provide a smooth transition between carbon dopantconcentrations, discreet layers of CDO having varying carbon dopantconcentrations may also be used to formulate a bulk interlayerdielectric. Moreover, the placement of the high and low carbon dopantconcentrations is variable dependent upon the types of structures beingformed and the types of capacitance and electrical fringing effectspresent. Thus, while specific examples of dopant concentration profileshave been provided, these dopant profiles are not meant to be limiting.

Thus, while several specific embodiments of the invention has beenillustrated and described, it will be appreciated that various changescan be made therein without departing from the spirit and scope of theinvention.

1. A process comprising: forming a metal structure having at least onevia and at least one interconnect onto a substrate, said metal structureextending above a surface of said substrate; forming, subsequent to saidforming said metal structure, a carbon-doped oxide (CDO) layer with afirst concentration of carbon dopants therein on said substrate andfilling entirely between elements of said metal structure; andcontinuing to form, subsequent to said forming said CDO layer with saidfirst concentration of carbon dopants, said CDO layer further above saidmetal interconnect structure with a second concentration of carbondopants therein, wherein said first concentration of carbon dopants ishigher than said second concentration of carbon dopants to form a hardmask with the second concentration of carbon dopants.
 2. The processaccording to claim 1 further comprising: forming, subsequent to saidforming said second concentration of carbon dopants, said CDO layerfurther with a third concentration of carbon dopants therein, whereinthere is a linear correlation of said concentration of carbon dopantsbetween said first concentration, said second concentration, and saidthird concentration.
 3. The process according to claim 1 furthercomprising: forming said CDO layer further with a third concentration ofcarbon dopants therein, wherein said first and third concentrations arehigher than said second concentration.
 4. A process comprising: forminga first layer of carbon-doped oxide (CDO) on a substrate, said firstlayer of CDO having a first concentration of carbon dopants therein;forming a second layer of CDO having a second concentration of carbondopants therein above said first layer of CDO; and forming a third layerof CDO having a third concentration of carbon dopants therein above saidsecond layer of CDO, wherein said first concentration and said thirdconcentration are higher than said second concentration to reduceelectric field fringe effects.
 5. A process comprising: forming a firstlayer of carbon-doped oxide (CDO) on a substrate, said first layer ofCDO having a first concentration of carbon dopants therein; forming asecond layer of CDO having a second concentration of carbon dopantstherein above said first layer of CDO; and forming a third layer of CDOhaving a third concentration of carbon dopants therein above said secondlayer of CDO, wherein said first concentration and said thirdconcentration are lower than said second concentration to reduceelectric field fringe effects.